Automatic telephone system



Original Filed Nov. M, 1930 Patented Feb. 2s, 1933 srarss rare JOHN I. BVELLAMY, OF BROOKFIELD, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS, TO n ASSOCIATED ELECTRIC LABORATORIES, INC., OF CHICAGO, ILLINOIS, A CORPORA- TION OF DELA'ALRE i AUTOMATIC TELEPHONE SYSTEM Application led November 14, 1930, Serial No. 495,591. Renewed March 7, 1932.V

This invention relates to automatic telephone systems in general, and more particularly to holding arrangements for use at the outgoing end of inter-office trunk lines which are of suciently lowresistance so that im' pulse repeaters` are not required.

-leretoforafon trunk lines of the above type, it has been the practice to omit the third, or holding, conductor and tol provide aholding device,commonly called a series or holdin@v repeater,` comprising a low resistance relay in Vone conductor of the trunk line. This relay is made slow acting and furnishes a holding potential to hold the preceding switches operated, or it may in turn control a slow-acting relay which furnishes-the holding potential.V Holding repeaters of this type are objectionable in that they increase the resistance of the trunk line and imbalance the impedance of the twoconductors of the trunk line, thereby decreasing the quality of impulse and voice transmission.

`lVhen the current iiow over a trunk line is discontinued after its seizure and before the connection is to be released, the series holding circuit will not function, as it depends upon the continued current iiow to hold it up. The current flow is discontinued during conversation in toll servicetrunk lines used in automatic telephone systems to extend connections from the toll board to the exchange in which the called line terminates. Such a toll service trunking system is disclosed in the patent to Wicks, 1,643,797, granted September 27, 1927. When the toll service trunking system is laid Vout so that incoming toll calls must be completed from trie toll boardto one or more exchanges by way of automatic switches located in an intermediate or tandem exchange, a third or holding conductoris employed in the trunk lines extending from the tandem exchange to the called. exchange to hold the switches in the tandem exchange, because a series holdi* device will notvfunction during conversation when there isno current flowing` in the trunk line. Y l Y The main object of the present invention, therefore, is the provision of an improved holdingV arrangement which eliminates the series relay does not adect the quality of I q impulse and vvoice transmissiony over the trunk line and which, moreover may be used in trunk lines wherein series holding devices will not function becauseof the discontinuation of the current ow at some stage of the connection.l l v The above object is attained in the present invention by bridging the grid-filament circuit of alvacuum tube across the two conductors of an outgoing trunk line and providing relays, controlled by the vacuum tube,

which furnish :the holding potential to the preceding switches from the time the trunk line is taken for usel until it is freed, at which time theholding potential is removed responsive to thefull potential of the exchange batterybeing impressed upon the grid-lilament circuit of the vacuum tube.V A clear understanding of the method of carryingoutthe above object may Vbest be had from the following detailed speciiication when read in connection withL the accompanying drawing,which shows by imeans of the usual schematic diagram a preferred embodiment of the invention.

Y Ink the drawing, the selector S is ofthe well known Strowger switch type having a directive vertical movement and an automatic rotary movement. fThe selector VS may beV accessible to other local selectors in the same exchange, lto incoming :selectors terminating trunks from other exchanges in cases where the exchange in which the selector S is located is used as a tandem exchange, or `directly to subscribers lineswitches. The selector S has ent invention, and serves to 4furnish `a holding potential to hold the selector S an-dfother preceding switches operated. 1

Having described iny general the apparatus Y shownl in the drawing, a detailed explanation its working contact completes a lockingl cir- 10 extending from ground, armature 28 andV its resting contact, armature 6 and its working contact, winding of release relay 10, to battery. Relay l0 operates and at armatur 1l and its working Contact places ground on the release trunkconductor' to hold the preceding switches in operated position. At armature 12 and its restingcontact it opens a point in the release circuit of the switchy and at the working Contact of this armature it preparesan impulsing circuit to the vertical magnet. Y

The calling subscriber now proceeds to dial the oilice designation of the desired exchange accessible over trunk lines, such as that comprising conductors 7 6 and 77. Besponsive to the dialling of the oce digit7 line relay 5`releases a correspondingnumber ci times and each time it deenergiaes' it closes a circuit from ground, armature 28 and its resting contact, armature 6 and itsresting contact, armature 12 and-its working Contact, winding of series relay 15, winding o1E vertical magnet 32 to battery. Series relay 15 operates in series with the `vertical magnet and remains operated throughout the series oic impulses. rlhe vertical magnet energizes at Veach impulse and raises the switch shaft and wipers' of the selector to a point opposite the level in which the trunk lines to the desired exchange are accessible. At the first vertical step of the shaft, vertical oil-normal springs 31 and 35 are closed. At vertical oil-normal Vsprings 31 a point is closed in the release, circrates in this circuit and at armature and cuit for itselt' independent of the contacts on the series relay 15, andat armature 22 and kits working contact prepares a circuit for the rotary magnet 38.

When the series relay 15VV deenergizeskat the end of the train of impulses, a circuit is closed for the rotary magnet'?) extending from ground, Working contact of armature 11 and said armature, armature 16 and its resting contact, armature 22 and its working Contact, winding ofrotary magnet 33 to battery. The rotary magnet energizes in this circuit and steps the shaft and wipers of the selector switch onto theirst set of contacts in the selected level.v Then the rotary magnet 33 energizes f it opens its interrupter Springs 34 thereby opening the circuit of stepping relay 2O and allowing this relay to release and at armature 22 and its working contact open the circuit of the rotary magnet. The rotary magnet releases and closes its interrupter springs 34C thereby againvpreparing a circuit orthefstepping relay 20.

The operation new depends upon whether or not the trunk line accessible in the first set of contacts in theselected level is busy. If this trunk Vlineis busy there will be ground potential present on the bank contact engaged by wiper 37 and this ground potential shunts tlie switching relay 25 and again completes the circuit of the stepping relay 2O over armature 29 and its resting contact, vertical oft-normal springs 35, interrupter springs 34 of the rotary magnet 33, winding of stepping relay 2O to battery. Relay 20 energizes and at armature 22-and its'wo'rking contact again completes the circuit to the rotary magnet, and the above operations are again repeated.

The selector switchis rotated in the above manner until the wipers encounter an idle trunk line. It will be assumed that the trunk line comprising conductors 76 and 77 and accessible to the selector switch over bank contacts'39, 40, and 41 is idle. Vlli/'hen the wipers of the selector switch are moved into engagement with these bank contacts, wiper 37 does not encounter ground potential and the switching relay 25 is no longer shortcircuited. As soon as the rotary magnet 33 ,deener'gizes and closes its interrupter springs 34 a circuit is completed for switching relay 25 extending from ground at the working contact of armature l1 and said armature, winding lof switching. relay 25, vertical ofi'- normal springs 35, rotary interrupter springs 34C, winding of relay 20 to battery. Relay 25 operates in this circuit but due to the high resistance of this relay, the relay 20 will not operate in series. At armature 28 and its resting contact, relay 25 removes ground from the armature of the line relay 5 in order to prevent the release of the switch when the line and release relays deenergize. At armatures 26 and 27 and their resting contacts, the line relay 5 is disconnected and at the working contacts of these armatures the calling subscribers loop is extended over wipers 36 and 38 and bank contacts 39 and 4:1 to the trunk line ext-ending to the desired ex# change. At armature V29 and its working contact the Vrelease trunk is extended to the holding trunk HTV over wiper 37 and bank contact 40. f

Relay deenergizes due toits disconnection from the calling loop at armatures 26y and 27, and at armature 6 and its working contact opens a point in the circuit of release relay l() and at its resting Contact closes a oint in the circuit of release magnet 30.

elease relay l0 is slow-to-release and maintains its armature in operated position for an interval after its circuit isopened. llien this relay finallyreleases it removes ground from the release trunk conductor at-.armature ll but this ground has been replaced by ground from the holding trunk HT, aswill be explained hereinafter. At armature l2 and its resting contact, relay 10 completes a further point in the circuit of release magnet 30.

The extensionl of the callingfsubscribers loop over wipers 36 and 38 and bank contacts 39 and 41l operates the line relay 80 of the incoming selector IS in the called exchange.

At the same time the ground potential extended over wiper 37 and bank contact 40 operates relay of the holding trunk HT. At armature 51 and its working contact, relay 50 closes the filament. circuit of the vacuum tube 70 connecting a portion of the secondary winding of transformer 6() to the lilament of this tube. The filament and plate currents for vacuum tube 7 0 are supplied through the transformer 60 from an alternating current Y the voltage drop inconductor 76 of the trunkV source over conductors 58 and 59. Y Normally with no connection to the trunk line over bank contacts 39 and 41, the full potential of the exchange battery at the distant exchange is impressed on the trunk conductor V76. The voltage of biasing battery line. The difference in the potential between points 7 8 and 7 9 with theV calling loop closed is therefore somewhat less than .the potential of the biasing battery`7l, and this battery therefore places a positive potential .on the grid of tube 70. lVith the .filament circuit of tube closed, as previously explained,

and a positive potentialonthe grid of this tube, a plate current lows from the plate of tube 70, winding of relay 65 and the resistc ance 67 in parallel, Vsecondary winding of transformer 60, V`to the filament of the tube.r This plate current operates the relay 65 which at armature 66 and its working contact connects ground tothe Windingof relayl 55 to operatethis relay. YRelay 55 at armature 56 and its workingV contact connects. ground vto the release trunk conductor, which servesl to lock therelay 50 in operated position and this .ground is also returned over bank contact-40 and wiper 37 to the release trunk conductor in the selector S to hold the yrela-y 25 of selector Sin operated position before release relay 10 of the selector has hadtime to As long as the calling loop remains closed, a positive potential is maintainedV on the grid of Vtube 7 O andthe relays 65, 55, and 50 are maintained'in their operated positions. yRe- -lay 55 maintains ground on therelease trunk conductor to hold all .the preceding switches in loperated position. Each. time thercalling loop is vopened during the subsequent dialling of the digits in the called telephone number,

a negative potential ismomentarily placed on the vgrid'of tube 70, thereby stopping the flow of plate current throughthe plate relay 65. Plate relay 65 is made slow-torelease by the resistance 67 in multiple with its winding so that it will not release during `the'negative half cycles of the alternating current in the secondary transformer 60, and this slow-release feature also tendstokeepy therelay op erated duringv the subsequent impulsing.

However the relay may not besuiiciently sloweacting to holdY upy duringfthek dialling offsubsequent digits andthe relay 55 is theres f fore provided with a resistance 57 in multiple with its winding so that this relay isalso made slow-to-release. gizes during impulsing. it opensthe. circuit of relay 55 momentarily, but since this relay i is also 'slow-to'release it will ynotV open its 'contacts `during the momentary interruption of this circuit. The two slow-to-release relays positively insure that ground willcnot be removed from the releasey trunk conductor during the operation of the switches in they called exchange.

`l/Vhenthe calling subscriber hangs up his receiver at the end of the conversation, the

calling loop is opened and the full negative potential of .the exchange battery in the called exchange is again impressed on the trunk'line conductor 76. This negative p0- tential overcomes the effe'ctaof the biasing battive with respect `to the 'Y filament, ythereby stopping the flow of kplate current through the plate relay 65. Relay'65 accordingly re-` leases, and at armature .66'an-d'its working contact opens the` circuit of the slow-torelease relay 55. After Aan interval relay 55 releases and at armature 56and its. working contactfopens the circuitief relay 50 and re- 10eA If lrelay 65 deenertery 7l and the grid'of tube 70 becomes negamoves ground from the release trunk conductor toallow the selector S to release. Relay j 50 upon deenergizing, at armature 5l and its working Contact opens the filament CrCll'lt ,l

,and its resting contact, armature 6 and its resting contact, armature l2 and its resting contact, vertical off-normal springs 31, winding of release magnet to battery. The release magnet 30 operates in this circuit and restores the switchrto its normal position.

When theswitch reaches its normal position the vertical off-normal springs 31 and 85 are again opened, the circuit of the release magnet being opened at vertical off-normal springs 3l. The removal of ground from the release trunk conductor by the release of relay also brings about the release of the preceding switches in theY same manner as explained with Vreference to selectors.

y In order to prevent any noise or hum from the alternating currentscurce due` to the capacity effect between the two windings of transformer from reaching the trunk line and interfering with the voice transmission, a high resistance 75 is inserted .between the connection of trunk conductor 77 to the filament of tube 70. This resistance may be as high as one megohin and serves to prevent any noise from reaching the trunk lines and also to prevent any unbalancing of the trunk lines. n A switch 72 is provided to connect the filament of tube either to conductor 77 through resistance 75 or to connect the filament of the tube directly to ground when the switch is operated to contact 74. y In case the difference in ground potentials betweenthe Y two exchanges is negligible the filament of tube 70 may be connected directly to ground and in this case a common transformer, such as 60, may beV used for all of the holding trunk circuits in the exchange. In this case the vacuum tube "i0y is controlled over conductor 76 with a ground'return circuit, instead of lover both conductors 76 and 77. Also the potential of the biasing battery V7l is preferably increased to about three-fourths of the voltage of the exchange battery, as 'only half the change formerly produced in the grid-ilamentvoltage is now produced when the trunk line is taken for use. Y 'Y From the foregoing, it is seen that the invention comprises V'a holdingl device which eliminates the third, `or holding, conductor of'interoflice trunk lines; which doesy not require any series relay or other resistance element in the talking conductors of the trunk line; and which is especially applicable to a system in which the current flow over the trunk line is discontinued at some stage of the connection, since the device is not dependent on current flow in the trunk line but is rendered operative as soon as the trunk line is seized and can be released only by the application of the full normal potential of .the exchange battery tothe conductors of the trunk line.

IVhat is claimed is:

l` In a telephone system, a trunk line, a

vacuum tube having its gridfilament circuit bridged across said trunk line, automatic 2. In a telephoneV system, a trunk line, a..

vacuum tubehaving its grid-filament circuit 'bridged across said trunk line, automatic switches for extending a calling line to said trunk, means .responsive to said extension for rendering said tube operative, means .fcon

trolled by said tube for maintaining said* switches operated, and means responsive to the opening of the calling loop for again rendering said tube inoperativethereby allowing said switches to release. 'y

3. In a telephone system, a. two conducto trunk line, a selectorl switch, means for'op-erating said selector switch to seize said trunk line, means including a vacuum tube responsive to said seizure for returning a holding potential to saidselector to hold the switch operated, and means responsive to the4 opening of the calling loop for removing said potential to allow said selector to release.

4;; In a telephone system, a trunk line, a selector switch having access thereto, a test wiper in said switch, a'vacuum tube associated with said trunk line, means for operating said switch to seize said trunk line, means controlled over said test wiper for rendering said tube operative, and means controlled by said tube for maintaining said last means and said switch operated.

5. In a telephone system, a two conductor trunk line connecting a first and a second exchange and' terminating 'in a battery in the second exchange, a vacuum tube in the first Y exchange having its grid circuit connected to the negative conductor of said trunk line whereby said grid is normally at a negativeV potential to render said tubeinoperative, means for extending a calling loop to said trunk line, and means responsiveto said extension for `reducing the effect of said battery to render said tube operative. Y

6; In a telephone system, a two conductor trunk line connecting two exchanges and terminating in the second exchange in a battery,

a. vacuum tube in tliefirstexchan e having its grid circuitconnected to the negative con'- ioo said trunk line, and means responsive to said"` extension for rendering vsaid tube operative,

the eiiective potential of said exchange battery being reduced to allow vsaid biasing battery to place a positive potential on said grid.

7.V In a telephone system, a trunk line con-v necting a first and a second exchange and terminating in a battery in the second exchange, a vacuum tube in the first exchange having its grid circuit connected to the nega-` tiveiconductor of said trunk line whereby said tube is normally rendered inoperative,

0 means for extending a callingVv line tosaid trunlrline,a biasing battery eective responsive to said extension for rendering said tube operative, and means controlled by said tube for Amaintaining said extending means opf 5 erated.

8. In a telephone system, a trunk line connecting aitirst and a second exchange, a vacnum tube in the first exchange having its grid cir-cuit connectedtothe negative conductor of said trunk line, means for extending a calling loop to said trunk line, a biasing battery effective when the calling loop is closed to render said tube operative, an exchange battery connected to said trunk line in the second exchange and effective to render'said tube inoperative when thecalling loop is opened, and means controlled by said `tube for supplying a holding potential to maintain said extending means operated while the calling loop is '10i closed and for removing said potential to allow said extending means to release when the calling loop is opened. y

9. In a telephone system, a trunk, an automatic switch having access thereto, la vacuum 5 'tube in said trunk rendered operative by current low over the trunk when it is seized by said switch, means controlled by said tube for supplying holding potential to saidV switch, and means for maintaining'said tube and holding means operative independent of current flow over said trunk.

10. In a telephone system, atrunlr terminating in a battery, an .automatic switch having access to said trunk, means in said trunk for Vsupplying holding potential. to

said switch, and a vacuum tube for operating said means responsive to current How over the trunk when it is seized lby said switch, said tube thereaftermaintaining said means operated independent of current ilow over said trunk until the full potential of' said battery is applied to said trunk.

1l. In a telephone system, a trunk terminating in a battery, anautomatic switch having access to said trunk, means in said trunk for supplying holding potential to said switch responsive to current low over the trunk when it is seized by said switch, and means for maintaining said holding means operated independent of current flow over,

said trunk, said-means being rendered inefectiveonly when the full battery potential is applied to said trunk. l,

121.- In a telephonesystem in which'automatic switches are employed to establish a telephone connection over a trunk line and in which all battery connections are removed Y holding circuit effective to ksupply holding potential to the switches used to vestablishV the connection, and meansgfor maintaining said holding lcircuit operative during conversation.

13. In a telephone system in which automatic switches are employed to v.establish a telephone connection'over -a trunk line and in which all battery connections are vremoved from the trunk line during conversation, y

means including` a vacuuml tube responsive to current flow. over 'said trunk line when-the same is `seized yfor supplying holding potential to said switches, said means remaining operative when thefbattery connections are removed from said trunlrline and releasing to discontinue the supplyiof` holding potential only when the battery connections lare again completed tosaid trunlr line at the end or the conversation.

14.) In a telephone System, a trunk, automatic switches for-establishing al connection i to said trunk, a holdingY circuit for said switches completed when the trunk is con-l nected with, a battery,;and means responsive to the application Aof the full batteryV potential to said vtrunk for opening said ,holding 1 circuit -tgo release saidswitches. v

15.In av telephone system, a trunk, a battery normally connected'to the conductors of said trunk, automatic switches for establish-` ing a connection to said trunkthe battery potential, on ythe conductors of said ,trunk4 i beingreducedwhen the connection is estabtalking conductors anda control conductor.

incoming to said device, means for establish- .i lished thereto, and means responsive tothe ing a-coniiection to said device over said three conductors,- a vacuum tube connected to said.

talking conductors, means controlled by said tube for controlling said connection over said control conductor, and means controlled over erative.

17. In a telephone system, a trunk,means for establishing a connection to said trunk, a vacuum tube having its grid-filament circuit bridged across the talking conductors of said trunk, means controlled by said tube for controlling said connection, and a high resistance connected between the lament circuit of the tube and one talking conductor ,of said trunk to prevent unbalance of said trunk and interference with conversation over said trunk. f

18. In a telephone system, a trunk, means for establishing a connection to said trunk, a Mholding circuit in said trunk including a va'cuum tube having its grid coupled to one talking conductor and its cathode coupled to the other talking conductor, and a high resistance connected between said'other talkming conductor and the coupling to said V cathode to prevent unbalance between said talking conductors and interference with conversation over said trunk.

19.. In`a telephone system, a'trunk ter-A Mminating in a battery, a holding circuit in 25"said trunk including a vacuum tube, means for establishing a connection to said trunk, a Y

"matic switch, means for operating said switch to connect .with said trunk line, and means bridged'acrossthe two conductors of said trunk line for controlling the supply of Mholding potentialto said switch, saidv means 40"neither supplying current'l to nor drawing current from the conductors oitV said trunk line. Y 21. In a telephone system, a two conductor Ntrunk line, an automatic switch for establish- 'inga connection to saidV trunk line, and a holding device in said trunk line for supplying holding potential to said automatic switch, the sole connections of said devices to said trunk line comprising a non-conductive 'bridge connected across the two conductors of said trunkline.

22. In a telephone system, a trunk coinprising two talking conductors, an automatic mswitch for establishing a connection to said trunk, a holding device in said trunk, athird conductor extending backrfrom said holding device to said automatic switch, and means in said device exclusive of any current con- `suming connections to the talking conductors 'of said trunk for supplying holding potential to said switch.

V23. In a telephone system, a trunk line comprising two metallic talking conductors @extending from the banks of an automatic `switch to a succeeding switch, holding equipment for thefirst switch individual to saidtrunk line, a third conductor extending back to said first switch romsaidequipment, a

switch to a succeeding switch,`holdingequipment for the'iirst switch individual 'to said trunk line, a third conductor extending back to said first switch from sai-d equipment, a vacuum tube included in saidL equipment and having itsV grid-filament circuit bridged across said trunk line, and means controlled by said tube forcontrolling'the'application of holding potential to said third`conductor. 25. In combination, a trunk line extending to an autcmaticswitch, means including 'a second switch for'extending a connection to said trunk line,'means in the first switch'ior supplying current over the trunk line to .a point ahead of said! second switch, and a p0- tentialY operated device connected to said trunk line between said switches and responsive "solely to potentialchanges on the trunk line which result from changes in the amount of current flowing from said first switch to said point for controlling the release of said second switch. y

- 26. In a telephone system, a trunk line comprising. two talking conductors extending exclusive of any series inductive windings or conductive bridges-from the banks of an automatic switch to a succeeding switch, a holding device individual to said trunk, a third conductor extending from said device back to said rst switch, means for supply-V ing current over the two talking conductors vfrom the second switch through the first switch to a calling bridge, anda potential operated device Yin said holding device re-4 sponsive solely to changes in potential on one of said conductors which result from changes in current iiow in the calling bridge for controlling the application of holding potential to said third conductor. j 'Y 27. In a. telephone system, a trunkv line comprising two talking conductors extending exclusive of any series inductive windings or conductive bridges from the banks of an auto,- matic switchto a succeeding switch, a holding device individual to said trunk, a third conductor extending from said vdevice. back to said first switch, means for supplying cur-- rentto the talking conductors of said trunk line from the second'switch, and means in said device responsive to a potential in one of said talking conductors for controlling the application of holding potential to said third conductor. Y

28. VIn a telephone system, a trunk rline to said third iso comprising two talking conductors extend'l ing excluslve of any series inductive Windings or conductive bridges from the banks of an automatic switch to a succeedingsvvitch.

a holding device individual to vsaid trunk, a

third conductor extending from said device back to said first switch, and means including a vacuum tube insaid device having its Vgrid connected to one talking conductor for controlling the application of holding potential to said third conductor. f

In Witness whereof, I hereunto subscribe my name this 8th day of November, A. D. 1930.

JOHN I. niiLLAMv.v 

